MANILA, June 11 (Reuters) – A Philippine province is almost certain to ban open pit mining due to environmental concerns, its governor said on Thursday, a move that could threaten development of one of Southeast Asia’s richest copper-gold mines.

Xstrata Plc’s $5.2-billion project is the centerpiece of the Philippine government's goal to attract billions of dollars in investments in its moribund mining sector. But residents fear the mining method to be used by Xstrata, which aims to start production in 2016, will pollute a major river used to irrigate farms.

Charlie Sartain, the chief executive of Xstrata Copper told Reuters in Santiago that the looming open pit ban by the South Cotabato province will not endanger the project.

Sartain said the Philippine mining codes support the projects.

“It is a serious announcement and we will take it very seriously, but at a national level within the Philippines we understand it is supportive,” he said.

The province’s legislative council approved the proposed ban under the province’s Environment Code on Wednesday.

The measure will soon be endorsed by the governor.

“The vote by the provincial council was unanimous so how can I veto it?,” South Cotabato Governor Daisy Avance-Fuentes, told Reuters in a phone interview. “I think I need to approve it.”

Avance-Fuentes, who has served the maximum three terms as governor, will be stepping down on June 30. She says the council’s decision would become law within 30 days from June 9 if there was no move from the governor. She also said the legislative council has the power to override her veto.

The state’s Mines and Geosciences Bureau said the looming ban “effectively means a ban on the development of the Tampakan copper-gold mine,” considered Southeast Asia’s largest undeveloped copper-gold prospect.

Peter Forrestal, president of Xstrata Copper’s Philippine affiliate Sagittarius Mines Inc., said studies showed open pit mining as the safest extraction method for Tampakan based on the configuration of the deposit and area’s geology. He added the group would monitor the issue closely and consider all options.

“The passing of this code is particularly disappointing given the significant stakeholder support shown for the Tampakan project during the public scoping meetings held late last year,” he said in a statement.

“SMI remains committed to the principles of sustainable development endorsed by this Code and believes that open pit mining methods are entirely compatible with these principles as demonstrated by the leading sustainable development practices used in all of Xstrata Copper’s open pit mines worldwide.”

The planned ban on open pit mining was originally proposed by provincial officials in 2005.

Xstrata has completed a feasibility study on the prospect and aims to start production in 2016. The mine, on the southern Mindanao island, has an estimated resource of 2.2 billion tonnes, containing 12.8 million tonnes of copper and 15.2 million ounces of gold at a 0.3 percent copper cut-off grade.